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I would clean it out with Vetericyn or betadine. You may want to give him a stronger antibiotic such as enrofloxacin in case he has E.coli in his abscess. You can search at the top of this page for "baytril or enrofloxacin sources" to find where to purchase it. Your local farm vet may also help. Can you get him to start roosting at night by placing him there after dark? Does he have any swollen leg joints or trouble walking?
Edited by country guy - 4/11/16 at 7:39pm
If you have had any respiratory problems such as MG or MS, one of the first signs can be a breast blister. But it sounds like his sleeping on the ground may be the problem. I had a cockerel who was in a large pen with 2 other more dominant fellows, and he developed a breast blister and scab from staying on a roost 24 hours a day. They wouldn't let him eat, drink, or walk around even, so I had to separate them, and eventually re-home them.
Can you post a picture of it?
Here is one I treated: